Blood on the Shoes: Presumptive Testing, Transfer, and Rain

What was done wrong, and why it matters:

• Blanket luminol spraying destroys information. Luminol is applied as a spray and reacts where it lands. Over 3 x 4 m it dilutes and diffuses trace material, can cause the reaction to run and blur boundaries, and makes it impossible to say which discrete stain reacted. It also complicates later confirmatory sampling. It should be used on discrete areas after visual examination, not as a lawn treatment.
• Pretreatment of a large area risks harming the pattern before it is documented. The blood impressions in the driveway should have been photographed with scale, measured, and cast or lifted before any reagent was applied.
• Luminol positives are not blood. Bleach, some paints, copper and copper alloys, rust, and various oxidising cleaning agents can produce chemiluminescence. A galvanised drain cover with rust streaks is exactly the sort of substrate that generates false positives, and the luminol field crossing that area is not evidence of a walk path.
• A single swab from the tread is inadequate. One sample from an unspecified location gives no spatial information and no control. There is no way to know whether the sampled area was the most concentrated, and no substrate control.

Better practice:

1. Document the scene visually first, with scale, before any reagent.
2. Apply presumptive tests (luminol, phenolphthalein/Kastle-Meyer, TMB) to discrete, targeted areas, with positive and negative controls and reagent blanks recorded.
3. Sample multiple discrete areas of the shoe - each tread groove separately, plus the midsole - and take substrate controls.
4. Send samples for confirmatory testing (crystal tests, immunochromatographic haemoglobin testing, and DNA) before drawing any conclusion about human blood.

Until confirmation returns, the correct statement is that brownish-red material is present; it is not yet established as blood.

What it can support:

• Material restricted to the tread grooves and midsole, with clean uppers, is the signature of contact transfer from a surface - standing or walking in a contaminated area. That is a different mechanism from being present during a blood-shedding event, which typically produces small droplets on the uppers, the toe box, and the side panels as well.
• The location within the tread (heel versus toe, outer versus inner edge) can indicate gait and which part of the sole loaded first, useful for correlating with the driveway impressions.
• The pattern and size of the tread can be compared with the crime-scene impressions. This comparison - tread design, wear marks, and dimensions - is far more probative than the mere presence of blood on a shoe.

What it cannot tell you:

• Whose blood it is. Only DNA or serology can address that.
• When the contact occurred. A shoe can pick up blood hours after an assault, from a contaminated surface, or from an unrelated source.
• Who wore the shoe, or that the wearer was the assailant. Someone arriving after the blood was shed would produce the same distribution.
• That the wearer walked through either of the two pools specifically, unless the pool's location and the impressions can be tied to that shoe by pattern comparison.

The honest formulation: the distribution is consistent with the shoe contacting a blood-contaminated surface; it does not establish identity, timing, or that the wearer was present during the assault.

Effect on the pools: 12 mm of rain has diluted and spread the margins, so the original extent and volume of both pools are unknown. The diluted margins should not be read as the original pool boundaries, and the amount of blood present cannot be inferred from what remains. On concrete, blood can wick below the surface and persist after the surface has been washed, so the best samples may come from the substrate rather than the surface film. A diluted, re-wetted pool can also produce edge rings and secondary satellite droplets that mimic spatter - another reason not to read fine droplets as impact evidence without checking.

Effect on the footwear trail: the impressions are degraded. Rain softens and rounds the margins, filling tread detail, so pattern comparison may be limited to general size and class characteristics rather than individual wear features. Continuity of the trail is also uncertain: apparent gaps may be washouts rather than an absence of impressions. The trail should be described as a series of impressions consistent with travel toward the street, with the interpretive limits stated, not as an unbroken path.

Timing question: establish whether the rain fell before or after the assault. If blood was still wet when it rained, dilution and runoff patterns may be rain artifacts, not the result of anything the suspect did.

Immediate priority: preservation. Rain is forecast again. Document and photograph everything with scale now, take samples from the deepest, least-exposed portions of the impressions, take substrate controls, lift or cast the clearest impressions before further rain, and cover what can be covered without contact. Sampling and documentation cannot wait for laboratory scheduling.